# Fiber Testing

//Fiber Testing

## Fiber Testing

What are the measurement units for power?
Optical power is measured in linear units of milliwatts (mW), microwatts (uW – really the greek letter “mu”W), nanowatts (nW) and decibels (dB).
What is the difference between “dBm” and “dB”?
dB is a ratio of two powers, for example the loss in a fiber optic cable. When power is measured in linear units (mW, uW or nW), dB is calculated on a log scale using this formula:
power (dB) = 10 log (power1/power2) If we are measuring absolute power levels, the measurement is generally referenced to 1 milliwatt (mW), is expressed as “dBm” and the calculation becomes: power (dBm) = 10 log (power/1 mW) Thus 1 mW = 0 dBm, 1 uW = -30 dBm, 1 nW = -60 dBm and two equal powers compared are 0dB (eg. power being the same, there is no loss.) What power level should a source have?
It depends on the type of source. When coupled into a good test cable, the source output power will be in these ranges:
LED: -10 to -25 dBm into 62.5/125 fiber
Telcom/LAN laser: 0 to -13 dBm into singlemode fiber, to +20 dBm with DWDM and fiber amplifier systems
CATV Laser : +16 to 0 dBm into singlemode fiber
What power level should a receiver see?
It depends on the network and type of source. When measured at the end of the network cable, the source output power will usually be in these ranges:
LAN/LED: -20 to -35 dBm into 62.5/125 fiber
Telcom/LAN laser: -20 to -45 dBm into singlemode fiber
CATV Laser : 0 to -10 dBm into singlemode fiber
How do you calculate a loss budget?The loss budget is a calculation of how much attenuation a link should have. You compare that loss to the dynamic range of the networking equipment to see if the range and link loss are compatible.
How accurate are fiber optic power meters?
All optical power meters which are calibrated to NIST (the US standards body) or any national standards lab will measure optical power to an uncertainty of about +/- 0.2 dB or 5%. Therefore, since every power meter has an uncertainty of +/- 0.2 dB, any two meters can differ by 0.4 dB in the worst case (one at +0.2 dB and one at -0.2 dB) even if both are within their specification! 